Use it everyday- since upgrading to ML, it doesnt seem to work, I can load my document (spreadsheets if what I typically use) but I cannot seem to edit the files.
I use Chrome but I just tried to edit a document on my Google Drive with Safari and everything was working fine. I also have Mountain Lion and the latest version of Safari.
Try doing things like emptying the browser cache, disabling extensions and trying with a different browser to try and determine the root of the problem.